Japanese patent laid-open publication No. Hei 4-19935 discloses a process of forming decorative wood laminates, such as sheets, by impregnating fine wood and cellulose powder with urea resin, drying and neutralizing the impregnated material, and entraining the neutralized material into a thermoplastic resin. Also, the publication discloses a method of forming a decorative wood board with the desired shape by additionally processing the decorative laminate, sticking the processed decorative laminate to a pallet having a cut network section along the thickness direction thereof, and pressing the resulting structure. However, the panel product has problems in that it can be used in a given specification due to a hollow in the center thereof, and if a portion thereof is cut and used, it will show deterioration in physical properties and a reduction in bending elastic modulus as a function of density, so that it will be unsuitable for use as a support for concentrated load. Another problem is that its production process is complicated.
Korean patent laid-open publication No. 96-443 discloses a form panel made of recycled resin, which is prepared by extruding recycled gel-like resin in the form of a plate, rolling the plate to the designed thickness while cold forming the plate into the desired shape. However, this product has problems in that it is heavy in weight, its physical properties are determined depending on the kind and composition of the recycled resin, and its production process is complicated.
Regarding compound panels, Japanese patent laid-open publication No. Hei 4-50900 discloses the technology of foaming the center layer of a multi-layer structure. The invention disclosed in this publication relates to an industrial multi-layer panel excellent in thermal resistance, impact resistance and rigidity, which is made of polyolefin, such as polypropylene or polystyrene. This panel is characterized by a multi-layer sheet of a glass fiber-reinforced resin composite, comprising a sheet layer of thermoplastic resin containing 10-45% by weight of glass short staple fibers, and a base sheet layer of expanded thermoplastic resin containing 5-50% by weight of fine inorganic powder and having a density of 0.4-1.11 g/cm3. However, this panel product has problems in that it is heavy in weight and low in impact strength since its center layer density and overall density are as high as 0.84-0.85 g/cm3 and 1.75-1.76 g/cm3, respectively.
Korean patent laid-open publication No. 2004-0003835 discloses panels capable of substituting for form panels for use in the formation of concrete walls, temporary door frames for wood door frames, stoppers and other plywood panels, as well as a manufacturing method thereof. However, the products disclosed in the publication are unsuitable for recycling products laminated with plywood as the coated products and shows a reduction in production efficiency upon the application of a pressing process. Also, this invention uses plywood, and thus, do not contribute to reduce the consumption of natural resources.
Korean patent laid-open publication No. 10-2002-0092590 discloses a synthetic resin panel manufactured from wastes, such as waste plastic, waste cotton, waste leather, waste wood, sawdust, etc., as main materials, as well as a form panel manufactured therefrom. This invention uses a glass fiber layer in which a continuous reinforcing fiber layer is non-impregnated, and thus, is weak in strength and interlayer adhesion. In addition, upon its use and cutting, fiber dusts fly in all directions, thus causing damage to the health of workers.
Korean patent laid-open publication No. 96-4300 discloses a multi-layer composite panel which has a hollow in the center and can be used as a concrete form. This panel is a five-layer panel manufactured by laminating a network fiber material on a hollow center layer of a lattice structure and sticking a foaming layer of composite resin on the laminated fiber layer by heating. However, the composite panel disclosed in the publication shows low physical properties since the adhesion between the resin layers is interfered by the network fiber layer. Also, it is difficult to separate the network fiber layer from the hollow center layer, in recycling, and due to the center hollow, the panel is weak against locally concentrated load or impact. Moreover, since the continuous reinforcing fiber layer is a non-impregnated glass fiber layer, it has no an effective reinforcing action and also the adhesion between the layers is weakened. Also, upon its use and cutting, fiber dusts fly in all directions, thus causing damage to the health of workers.
Korean patent laid-open publication No. 10-2001-0016955 discloses a multi-layer panel comprising a surface layer treated with synthetic resin on both or single side thereof, and a center layer made of a mixture of wood flour, chaff powder and waste plastic resin. This panel is lighter in weight than a multi-layer resin panel and satisfies the requirements of both strength maintenance and water tightness. However, it has no reinforcing fiber layer, and thus, is insufficient in bending strength, physical properties, linear thermal expansion coefficient, and impact strength.
In addition, wood panels which are used for industrial applications and as form panels in the constructional field have not only limitation in their supply but also the risk of damage to forests. For this reason, the development of a substitute material for the wood boards now exists. Accordingly, various plastic and synthetic resin-based panels are currently developed. The common disadvantage of the plastic panels is that the bending elastic modulus at temperature of 30-60° C. is low. Also, their liner thermal expansion coefficient is generally as high as 1×10−4/K, and thus, if there is a great change in surrounding temperature, the panels will encounter problems caused by their expansion or shrinkage. Although these problems can be solved to some extent by increasing the content of a filler and the like, the increase in the filler content results in a reduction in impact resistance and an increase in density leading to an increase in weight, thus making the use of the panels inconvenient. These problems are caused by the intrinsic properties of commercially available thermoplastic polymers, and thus, unavoidable in products manufactured from such materials. On the other hand, wood panels have a very low thermal expansion coefficient of 1×105/K at a temperature of 0-50° C., and a high bending elastic modulus of 25,000-50,000 kg/cm2. Also, they show an insignificant reduction in physical properties at high temperature and have a lightweight corresponding to a density of 0.6-0.8 g/cm3. However, the existing products fail to satisfy all such requirements, and thus, cannot fully substitute for the functions of wood panels.
In attempts to solve these problems, any of the following methods as done in the prior art is used: (1) a glass fiber-reinforced resin composite layer is used; (2) a network sheet of glass fibers is laminated; (3) a composite resin layer containing a significant amount of wood flour is used; or (4) a wood board is coated with a certain material. However, the method (1) provides a product reinforced with short staple fibers, and thus, results in limitations in improving bending strength and linear thermal expansion coefficient. This limitation may also be overcome by increasing the amount of reinforcing fibers or the aspect ratio of fibers, but in which case manufacturing cost will be increased. The method (2) including the use of the network layer of glass fibers hardly exhibits sufficient effects since the impregnability of thermoplastic resin into glass fibers is very poor. The method (3) including the use of a large amount of wood flour and inorganic filler results in a great reduction in material cost, but shows no great effect on the improvement of physical properties. Thus, this method is generally used in combination with the method (1). The method (4) cannot basically solve environmental burdens caused by the use of wood boards, i.e., environmental destruction caused by felling. Also, the method has the problem of interlayer separation since the adhesion compatibility between the wood board and the resin layer is low.
Multi-layer panel products including thermoplastic polymer resin as a base resin mostly have a linear thermal expansion coefficient of more than 8×10−5/K. For this reason, there is a felt need for the development of technology allowing shrinkage and expansion to be remarkably low even in the case of a severe change in temperature, such as the daily range of temperature, while making impact and bending strength excellent.
For automobile parts, various composite resin materials are now being developed to solve the problems of cost and weight in the existing metal and steel materials. Particularly, a compound material for a bumper back beam is manufactured by the technology comprising laminating a glass fiber mat, chopped fibers and thermoplastic polyolefin resin (e.g., polypropylene) and melt-compressing the laminate to prepare a glass mat thermoplastic (GMT) sheet, heating and compression-molding the GMT sheet into the desired shape. Although this technology provides relatively high specific strength and impact strength, the method of making the GMT sheet is a heat-compression method providing only incomplete impregnation of the resin into the fibers. Thus, this technology has problems in that not only a non-impregnated portion of the fibers does not effectively contribute to enhance the physical properties of the product, but also an environmental problem, such as dropping of fibers onto workers in drilling and bolting operations, is caused.
Also, since a simple structure, sufficient melting time and compression time are required, in order to achieve sufficient impregnation in the compression-molding process, this technology has the problem of an increase in molding time and cannot form ribs with various structures.
In addition to the GMT technology, there is suggested a method of manufacturing completely impregnated long fiber thermoplastic composite products by a pultrusion process. However, the products manufactured by this method have a problem in that they are insufficient in strength or cause defects at welded lines, due to insufficient fiber length.